The present invention was conceived initially in the context of magnetic lifting devices, but as will become evident from the below description, it has applications beyond devices for hoisting ferromagnetic materials and work piece holders. Development of the invention was effected in the context of permanent magnets but it is believed that the underlying principles are transferable to magnet arrays that employ electromagnets.
Magnetic lifters are versatile material handling devices that use magnetic force to attach one or more ferrous material work pieces, ranging from small bundles of rod or scrap material to large heavy blocks or sheets of ferromagnetic materials, to a contact face of the device, thereby allowing transport of the work piece from one location to another whilst being securely held by the device.
Magnetic lifters can either utilize electro-magnets, which allow for adjustment of the magnetic field and thus the pulling force exerted onto a work piece at the contact face of the lifter device, or employ permanent magnets which are held in a movable rotor (or other support structure) within a housing so as to be selectively brought into interaction with passive pole pieces that abut at (or provide) the work piece contact face of the device, ie the contact face may be devised to act as a passive pole piece for the magnet(s) such that direct contact between magnet(s) body and work piece is avoided to prevent environmental contamination of the magnet(s) or operational difficulty in separation of the work piece from the magnets.
Modern permanent magnet lifters, in general, utilize permanent magnets which generally produce a high intensity magnetic field. Advances in metallurgy and magnetic technology in the last decades have resulted in the availability of magnetic materials with unprecedented power—most notably “Rare Earth” magnets, some of which exhibit a pulling strength of more than 100 times their own weight. They do not suffer significantly from problems like degrading over time or sudden loss of magnetic power due to exposure to moderate external magnetic influences or the removal of keepers, as ‘traditional’ permanent magnets tend to suffer. Permanent magnet lifters having low dead weight and lifting capacities from 100 to 2000 Kg have thus been introduced into the market place.
Examples of permanent magnet lifting devices which allow manual activation and deactivation of the lifter are those manufactured and sold by the Italian company Tecnomagnete under their RD modules, SMH module, and MaxX and MaxX TG Series.
A turn-off permanent magnet for use as a lifter is disclosed in U.S. Pat. No. 3,452,310 (Israelson). There, a stack of ceramic plate magnets (providing a first N-S dipole structure) is held sandwiched at an upper end of and between rectangular, plate-like pole pieces which provide at their lower free ends the working air gap for attachment to a ferromagnetic work piece. An armature consisting of a stack of ceramic plate magnets (providing a second N-S dipole structure) with segment-shaped pole pieces at each stack end is held rotatably within a cylindrical zone defined between and extending into the plate-like pole pieces, whereby the rotational position of the armature will either augment the magnetic field at the pole piece working faces (i.e. the N and S poles of the armature coincide with the N and S poles which the first dipole structure imparts to the pole pieces) or effectively shunt the magnetic field of the upper magnet stack by providing an internal closed loop magnetic path between the dipole structures.
U.S. Pat. No. 4,314,219 (Haraguchi) describes a somewhat similar concept, wherein a plurality of rotatable armatures consisting of stacked plate-like permanent magnets are disposed in an array within cylindrical cavities defined between a plurality of (magnetisable) passive magnetic poles encased within an outer non-magnetiseable housing. Here again, rotational position of the armatures will dictate the magnetization state of the pole pieces which are used to provide an external flux path when the pole piece working faces abut on a work piece.
These types of lifters produce in their active state in general a fixed magnetising force which is directly related to the magnetic length of the particular design. Magnetic length is defined as the distance between pole pieces in between which is received a volume of active magnetic material, eg the length between opposite polarity end faces of a dipole magnet. The output of magnetic energy is dependent on the amount of active magnetic material and its type, thus essentially a fixed value. However, in situations where the work load cannot absorb all magnetic energy provided by the magnet, the pulling force on an attached object is reduced. The surplus magnetic energy presents itself as leakage with associated magnetic stray fields.
Whilst factors concerning load carrying capacity are mostly properly addressed in existing devices, problems remain.
A particular problem exists in magnetic lifter applications where it is necessary to lift single metal sheets from a stack of such sheets. Existing devices are primarily configured for weight lifting capacity and will have a contact surface that enables planar attachment to the upper most sheet in a stack. However, such lifters will be unable to lift in a discrete manner a single sheet from the stack unless an air gap of sufficient height between the upper most and the next sheet in the stack is maintained, or the relative position of the permanent magnets employed to ‘switch’ the device on and off is chosen to assume an ‘intermediate’ state where the magnetic flux density available at the pole piece faces that engage with the work piece is reduced, with a consequential drop in the magnetic pulling force. The same considerations apply to electromagnetic lifters when the electric current is reduced to allow for sheet separation and avoidance of magnetic field penetration into adjoining sheets.
In the case of permanent magnetic lifters, when the pole pieces, which are in contact with the permanent magnets, are brought with their working surfaces into contact with the upper most metal sheet, a closed or loaded magnetic circuit is created. Unless the (magnetic) permeability of the sheet material and thickness of the sheet are such that the (external) magnetic flux path created is fully confined within the upper sheet, and no leakage (le a flux path outside the intended magnetic circuit comprising the magnet(s), pole pieces and upper sheet alone) spills into the adjoining next sheet, the lifter device will tend to lift such number of sheets which are magnetically attached together, as determined by the maximum weight lifting capacity and penetration of the magnetic field of the magnet(s) into the stacked sheets. In other words, if the uppermost metal sheet can not carry the whole magnetic flux provided by the magnet(s), flux over-saturation will occur in the upper most sheet, and the magnetic field will extend beyond the thickness of the upper most sheet into the lower next sheet(s) to an extent where saturation of a lowermost located sheet is no longer present; the magnetizing force in effect will magnetically clamp a number of sheets together for lifting by the lifter device.
A typical approach to deal with the single sheet lifting problem is described in US patent application publication US 2005/0269827 A1. This document describes a permanent magnet lifting system which employs as Integral components on a frame a plurality of shallow-field magnetic devices specifically devised to allow lifting off single ferromagnetic sheets from a stack of sheets.
A plurality of magnetic lifting devices is arranged in a two-dimensional array, eg 4×2 rectangular array, to engage the sheet at multiple locations over the sheet's top surface area. Importantly, the individual lifting devices are spaced apart to such an extent that no interaction takes place between the respective magnetic fields and fluxes which each of the devices generate when in contact with a metal sheet.
To limit the penetration depth of the magnetic field of each magnetic device, permanent magnets with short and fixed magnetic length are used. In order to increase overall volume of active magnetic material and achieve the desired lifting capacity, a plurality of such individual short length magnets are connected in series to provide a single magnetic field orientation, ie each device is comprised of a stack of permanent magnet plates (magnetised in the thickness direction of the plate such that opposite faces have opposite polarities) interleaved with soft iron pole piece plates. The magnet plates are arranged alternately with faces of equal polarity opposing one another across the intervening pole piece, such that a series of alternating North-South-North-etc. magnetic fields along the stacking direction are present between pole pieces, neighbouring pole pieces thus providing a plurality of working (air) gaps along the stacking direction. That is, the active magnetic material of each device is subdivided into discrete portions and interleaved and in contact with passive magnetic material, thus creating a plurality of shallow magnetic field loops between the pole pieces.
One immediately apparent problem with the lifting frame of this US patent document is that the magnet devices can not be switched off, and mechanical levers are used to forcibly disengage the sheet from the frame when required. Because the stacked row of individual short magnetic length magnets generate an overall uniform large flux in a common direction in an attached work piece sheet, the latter will be prone to remanence problems (residual magnetisation in the detached work piece).
It is one object of the present invention to provide in one aspect thereof, a lifter device which utilizes permanent magnets as a source of a magnet field intended to interact with ferromagnetic sheet material, and which device can be switched between ‘on’ and ‘off’ states, the ‘on’ state enabling discrete lifting of individual sheets from sheets stacked without a substantial air gap between neighbouring sheets.
It is another object of the present invention to provide in another aspect thereof, a configuration/arrangement of discrete magnetic field sources which overall generates an effective attraction force between a device incorporating the arrangement and a work piece and which simultaneously enables substantial confining of magnetic flux lines generated by the arrangement in the work piece upon an external magnetic circuit being created therewith.
Yet another object of the invention is to provide in another aspect thereof, a configuration/arrangement of discrete magnetic field sources which generates an effective pulling force between a device incorporating the arrangement and a work piece in which the pulling force exerted on the work piece is larger than the pulling force which the sum of the individual magnetic field sources would have.
Yet another object of the Invention is to provide in another aspect thereof, a configuration/arrangement of discrete magnetic field sources in a magnetic circuit which generates an effective pulling force between a device incorporating the arrangement and a work piece and in which the magnetic flux transfer is not unilaterally dictated by the magnetic field sources but wherein an autonomous internal magnetic flux regulation takes place to match the magnetising force of the flux source to the ferromagnetic saturation properties of an external load provided by the work piece.